Thoughts on the most difficult instruments for speakers to reproduce?

Compare the frequency (both high/low) and dynamic (SPL) ranges of piano (or- any other instrument) and pipe organ, here: https://www.zytrax.com/tech/audio/audio.html#frequencies      Not a matter of opinion (all other considerations being equal)!

@isochronism Still worked for me!

I was listening to a nice, well regarded pair of speakers and getting ready to pull the trigger when on a whim I asked to hear an older 70's rock recording and that's when the speakers tweeter showed its true nature and the salesman couldn't stop the playback fast enough. Resale on new speakers is horrible so when buying new play everything you can and listen to how the system handles less than stellar recordings, listen for compression and how the speakers handle everything. The harder  it is to hear the compromise, the longer they'll last in your home.

Steve...glad you were able to avoid an expensive mistake. Tweeter attributes are critical in speaker selection. Don’t laugh but I would always bring the ultimate tweeter killer speaker shopping with me, BST Spinning Wheel. The opening horns exposed serious flaws in many expensive speaker tweeters . Ultimately I went with  soft domes which is a trade off but worth it to me....not that those BST horns will ever sound good on any speaker :)

Its not just speakers. These things are difficult for every component in the chain. As my system has gotten much better over the past year or so one of the best most unexpected surprises has been the way every recording sounds so much better now. Every single one. So if you have one that's a "tweeter killer" I would look long and hard at what else might be going on there.